1. Field of the Invention
The present invention relates to a distortion compensating amplifier that compensates for distortion occurring in an amplifier that amplifies a signal, and, more particularly to a highly efficient distortion compensating amplifier that realizes highly accurate distortion compensation.
2. Description of the Related Art
For example, in a base station apparatus and a relay station apparatus for mobile communication, a multi-carrier signal including a large number of carrier waves, which are properly modulated at predetermined frequency intervals, respectively, is transmitted by radio after being subjected to high-frequency amplification. When linearity of an amplifier used for the high-frequency amplification is not sufficiently high, various kinds of distortion such as intermodulation distortion occur. This distortion hinders realization of normal and high-quality communication. Therefore, an amplifier used for amplification of a multi-carrier signal is required to have high linearity over an entire frequency band to which the multi-carrier signal belongs.
As one method of realizing an ultra-low distortion amplifier suitable for amplification of a multi-carrier signal, there is a feed forward (FF) amplification system.
For example, a signal path extending from a signal input terminal to a signal output terminal through a main amplifier, that is, a signal path for transmitting a signal that should be amplified and an amplified signal, is called a main line. In the FF amplification system, a distortion detection loop for connecting a signal branching from a certain point at a post-stage behind the main amplifier on the main line and a signal branching from a certain point in a pre-stage before the main amplifier on the main line is provided. If electrical lengths of signal paths through which both the signals are transmitted are equal to each other and both the signals have opposite phases at the same amplitude, it is possible to extract a signal equivalent to distortion occurring in the main amplifier and peripheral circuits thereof by canceling carrier wave components according to an operation of the signal combination.
In the FF amplification system, a distortion compensation loop is further provided to recombine the signal extracted in the distortion detection loop, that is, the signal equivalent to distortion, with the signal on the main line. When a signal delay in the distortion compensation loop is compensated on the main line and adjustment of an amplitude or a phase is performed in the distortion compensation loop or the main line such that distortion components included in the signal on the main line and a signal obtained from the distortion compensation loop have opposite phases at the same amplitude, it is possible to compensate for the distortion occurring in the main amplifier according to an operation of the signal recombination.
FIG. 5 is a diagram of an example of a circuit configuration of a feed forward amplifier (FF amplifier).
In the FF amplifier in this example, a distortion detection loop L1 and a distortion compensation loop L2 are formed using three hybrids 61, 66, and 71. In FIG. 5, a signal path extending from a signal input terminal IN to a signal output terminal OUT through a main amplifier 64 and a coaxial delay line 67 is a main line. A signal path extending from the input terminal IN to an output terminal of the hybrid 66 through a coaxial delay line 65 is the distortion detection loop L1. A signal path extending from the output terminal of the hybrid 66 to an output terminal of the hybrid 71 through an auxiliary amplifier 70 is the distortion compensation loop L2. Dummy loads 81 and 82 have impedances equal to a characteristic impedance of the line and are used for a trailing end of a terminal of the hybrid 61 and a trailing end of a terminal of the hybrid 71, respectively. The main amplifier 64 and the auxiliary amplifier 70 are constituted by combining, for example, plural amplification elements 91a to 91d and plural amplification elements 92a to 92d, respectively.
When, for example, a multi-carrier signal is applied to the signal input terminal IN as a signal, this signal is inputted to a variable attenuator 62 and a variable phase-shifter 63 via the hybrid 61. The signal is subjected to adjustment of an amplitude and a phase by the variable attenuator 62 and the variable phase-shifter 63 and amplified by the main amplifier 64. The signal amplified by the main amplifier 64 is inputted to the hybrid 66 and, at the same time, inputted to the hybrid 71 via the coaxial delay line 67. The signal is outputted to a circuit at a post-stage from the hybrid 71 via the signal output terminal OUT. The coaxial delay line 67 is a delay line for compensating for a signal delay occurring in, in particular, the auxiliary amplifier 70 that is a circuit constituting the distortion compensation loop L2.
A signal inputted from the signal input terminal IN is divided into two signals by the hybrid 61. The two divided signals are the same in terms of a frequency configuration of components. Whereas the divided signal supplied to the main line side is amplified by the main amplifier 64, the divided signal supplied to the distortion detection loop L1 side is supplied to the hybrid 66 from the hybrid 61 via the coaxial delay line 65 while generally keeping an amplitude thereof. The coaxial delay line 65 is a delay line for compensating for a signal delay occurring in, in particular, the main amplifier 64 that is a circuit on the main line side. The signal supplied to the hybrid 66 via the coaxial delay line 65 is combined with a signal including distortion components by the hybrid 66.
The hybrid 66 divides the signal including the distortion components outputted from the main amplifier 64 into two signals. The two divided signals are the same in terms of a frequency configuration of components. One divided signal is supplied to the main line side and the other divided signal is supplied to the distortion compensation loop L2 side. In supplying the other divided signal to the distortion compensation loop L2, the hybrid 66 combines this signal and the signal supplied through the coaxial delay line 65 to thereby extract the distortion components from this signal while canceling carrier wave components in this signal.
A signal obtained as a result of this combination is supplied from the hybrid 66 to the variable attenuator 68, the variable phase-shifter 69, and the auxiliary amplifier 70 constituting the distortion compensation loop L2. The signal is subjected to adjustment of an amplitude and a phase by the variable attenuator 68 and the variable phase-shifter 69, amplified by the auxiliary amplifier 70, and inputted to the hybrid 71. The signal inputted to the hybrid 71 is combined with the signal supplied through the coaxial delay line 67 in the hybrid 71. Consequently, the distortion is canceled and an amplified signal after distortion compensation is outputted from the signal output terminal OUT.
In order to cancel the carrier wave components and extract the distortion occurring in the main amplifier 64 and the like by combining the divided signal of the output signal supplied from the main amplifier 64 and the signal supplied through the coaxial delay line 65, a predetermined number of carrier wave components included in the divided signal of the output signal supplied from the main amplifier 64 and the same number of carrier wave components included in the signal supplied through the coaxial delay line 65 are required to have the same timing, the same amplitude, and opposite phases at the point of the combination in the hybrid 66. The coaxial delay line 65 is means that sets carrier wave components at the same timing.
As another example, there is a cross-cancel system that is a distortion compensation system higher in efficiency than the FF amplification system. Briefly, the cross-cancel system is a system in which the main amplifier 64 of the FF amplification system shown in FIG. 5 is divided into two amplifiers to combine main signals and cancel only distortion components. Compared with the FF amplification system shown in FIG. 5, since the auxiliary amplifier 70 for amplifying distortion components is unnecessary, the cross-cancel system has higher efficiency.
An example of a circuit configuration of an amplifier of the cross-cancel system is shown in FIG. 6A.
The cross-cancel system is different from the FF amplification system shown in FIG. 5 mainly in that the main amplifier 64 is divided into two amplifiers and used as a first main amplifier 104 and a second main amplifier 110 instead of the main amplifier 64 and the auxiliary amplifier 70. The auxiliary amplifier 70 used in the FF amplification system is not used. The cross-cancel system is also different from the FF amplification system in that a 3 dB coupler or a combiner is used as a hybrid 111. The main amplifiers 104 and 110 are constituted by combining, for example, plural amplification elements 131a to 131c and plural amplification elements 132a to 132c, respectively.
In the amplifier of the cross-cancel system in this example, the distortion detection loop L1 and the distortion compensation loop L2 are formed using three hybrids 101, 106, and 111. In FIG. 6A, a signal path extending from the signal input terminal IN to the signal output terminal OUT through the first main amplifier 104 and a coaxial delay line 107 is a main line. A signal path extending from the signal input terminal IN to an output terminal of the hybrid 106 through a coaxial delay line 105 is the distortion detection loop L1. A signal path extending from the output terminal of the hybrid 106 to an output terminal of the hybrid 111 through the second main amplifier 110 is the distortion compensation loop L2. Dummy loads 121 and 122 have impedances equal to a characteristic impedance of the line and are used for a trailing end of a terminal of the hybrid 101 and a trailing end of a terminal of the hybrid 111, respectively.
An example of an operation in the cross-cancel system is described below.
When, for example, a multi-carrier signal is applied to the signal input terminal IN as a signal, this signal is inputted to a variable attenuator 102 and a variable phase-shifter 103 via the hybrid 101, subjected to adjustment of an amplitude and a phase by the variable attenuator 102 and the variable phase-shifter 103, and amplified by the first main amplifier 104. The signal amplified by the first main amplifier 104 is inputted to the hybrid 111 via the hybrid 106 and the coaxial delay line 107. A signal on the main line generated at this point is set as A (in this explanation, representing a vector) and distortion components at this point are set as B (in this explanation, representing a vector). The coaxial delay line 107 is a delay line for compensating for a signal delay occurring in, in particular, the second main amplifier 110 that is a circuit constituting the distortion compensation loop L2.
A signal inputted from the signal input terminal IN is divided into two signals by the hybrid 101. The two divided signals are the same in terms of a frequency configuration of components. Whereas the divided signal supplied to the main line side is amplified by the first main amplifier 104, the divided signal supplied to the distortion detection loop L1 side is supplied from the hybrid 101 to the hybrid 106 via the coaxial delay line 105 while generally keeping an amplitude thereof. The coaxial delay line 105 is a delay line for compensating for a signal delay occurring in, in particular, the first main amplifier 104 that is a circuit on the main line side. The signal supplied to the hybrid 106 via the coaxial delay line 105 is combined with a signal including distortion components by the hybrid 106.
The hybrid 106 divides a signal including distortion components outputted from the first main amplifier 104 into two signals. The two divided signals are the same in terms of a frequency configuration of components. One divided signal is supplied to the main line side and the other divided signal is supplied to the distortion compensation loop L2 side. In supplying the other divided signal to the distortion compensation loop L2, the hybrid 106 combines this signal and a signal supplied through the coaxial delay line 105 to thereby cancel carrier wave components in this signal and extract the distortion components from this signal.
A signal obtained as a result of this combination is supplied from the hybrid 106 to the variable attenuator 108, the variable phase-shifter 109, and the second main amplifier 110 constituting the distortion compensation loop L2. The signal is subjected to adjustment of an amplitude and a phase by the variable attenuator 108 and the variable phase-shifter 109, amplified by the second main amplifier 110, and inputted to the hybrid 111. A signal on the main line at this point is set as C (in this explanation, representing a vector) and distortion components at this point are set as D (in this explanation, representing a vector).
Since the first main amplifier 104 and the second main amplifier 110 are the same amplifiers, amplitudes of the main signal and the distortion components are fine-tuned by the variable attenuator 108 to be the same.
An example (i) of spectra of a signal after being amplified by the first main amplifier 104, an example (ii) of spectra of a signal after being amplified by the second main amplifier 110, and an example (iii) of spectra of a signal after being combined by a coupler serving as the hybrid 111 are shown in FIG. 6A.
In the examples, |A vector| is equal to |C vector|. The vectors are amplitude components of the main signal. |B vector| is equal to |D vector|. The vectors are amplitude components of the distortion components.
A phase of the A vector is set as θ1, a phase of the B vector is set as θ2, a phase of the C vector is set as θ3, and a phase of the D vector is set as θ4.
In this example, phases of respective main signal components are adjusted to the same phase to combine the main signal components. In other words, θ1 and θ3 are set to be equal.
In this example, phases of the distortion components are adjusted to opposite phases to cancel the distortion components. In other words, for example, θ2 is set equal to θ4—180 degrees.
An example of a state of combination of distortion components is shown in FIG. 6B.
JP-A-2004-15506 is a patent document describing the technique described above.
In the past, for example, distortion compensation by the cross-cancel system higher in efficiency than the FF amplification system has been examined as described above. However, in the conventional cross-cancel system, it is impossible to control a signal and distortion components on the main line separately. Thus, it is extremely difficult to sets main signal components in the same phase and set the distortion components in opposite phases.